Gaseous waste products from the combustion of fuels pose serious health and environmental problems. Exhaust gases from hydrocarbonaceous fuel-burning sources such as stationary engines, industrial furnaces, industrial processes, etc., contribute significantly to air pollution, and the exhaust gases of automobile internal combustion engines have been recognized as a principal source of air pollution. In recent years there has been increasing interest, in view of the large number of automobiles traveling our roads, particularly in urban areas, in controlling the amount of gaseous waste products from automobile exhaust.
Automotive catalytic converters containing exhaust gas catalysts have more or less enabled automobiles to meet current standards established by government agencies to convert a substantial portion of hydrocarbons and carbon monoxide to water and carbon dioxide and the NO.sub.x gases to nitrogen and oxygen and/or water. A wide variety of metals and metal oxides, either alone or in combination, supported on various substrates have been utilized. In recent years, most exhaust gas catalysts have employed a combination of noble metals, particularly platinum, rhodium, and/or palladium, as the active materials of the catalyst. Often, the activity of the noble metals is enhanced by the addition of small amounts of other metals which act as promoters.
Typically, exhaust gas catalysts comprise a relatively low porosity ceramic support with a transition alumina coating having a high surface area. The underlying ceramic support is generally prepared by sintering a mold of clay or other ceramic material at a high temperature to impart density and strength. This, however, generally results in a support having a very low surface area. Consequently the ceramic support must be coated with another material having a much higher surface area to contain the noble metals. The procedure of depositing a high surface area "washcoat," as such coating is generally known, onto a low surface area ceramic support is disclosed in, for example, U.S. Pat. Nos. 2,742,437 and 3,824,196. The ceramic supports may be provided in any shape, but typically they are in the form of pellets or a honeycomb-type shape commonly known as a monolith.
Gamma-alumina is often used as the washcoat in such exhaust gas catalysts. Although a gamma-alumina washcoat imparts a relatively high surface area to an exhaust gas catalyst, it results in number of undesirable effects. Often the washcoat does not adhere well to the underlying ceramic support under severe thermal stress, or has a level of thermal expansion incompatible with the ceramic support. In addition, gamma-alumina or other transition-alumina washcoats are thermodynamically unstable alumina phases. Eventually this unstable phase transforms to a thermodynamically stable alpha-alumina phase; however, in the process of transforming, the alumina loses surface area and traps noble metals, particularly rhodium, and may change their oxidation state, rendering the noble metals less effective or ineffective. Accordingly, the in situ conversion of an impregnated gamma-alumina carrier to the alpha form does not result in a catalyst of this invention.
Use of relatively large amounts of precious noble metals is a further drawback of conventional washcoated exhaust gas catalysts. This, coupled with the problem of entrapment of the noble metals under thermal stress, makes reclamation of the noble metals from these catalysts difficult. Costly, uneconomical reclamation techniques typically are required that, at best, are able to reclaim only a portion of the noble metals from used catalysts. Only about 5% of the total automobile exhaust gas catalysts employed worldwide are subjected to reclamation. The percentage of noble metals recovered from these catalysts is typically in the range of 60-80% for palladium and platinum and 50-60% for rhodium. As a consequence, the prices of noble metals have risen to very high levels in the last decade.
Conventional washcoated exhaust gas catalysts also require a time-consuming, tedious, cost-ineffective, multi-step preparation procedure. This procedure includes preparation of the support, preparation of the washcoat itself, including impregnation of all the catalytic and promoter components individually or collectively into the washcoat, and application of the washcoat onto the support.
Although washcoated exhaust gas catalysts have acceptable initial light-off temperatures, with age their light-off temperatures often increase, sometimes rapidly. Light-off temperature ("T.sub.50 ") is the temperature at which an exhaust gas catalyst begins to convert 50 percent of the waste products of the exhaust gas into carbon dioxide, water, nitrogen and oxygen. Thus, when an automobile is initially started and for the time until the catalyst reaches its light-off temperature, most of the exhaust gases are not catalytically treated but are simply emitted into the atmosphere.
Stable catalytic activity is becoming a critical requirement with automotive exhaust gas catalysts. Conventional washcoated exhaust gas catalysts lose approximately half of their activity relatively rapidly, i.e., during the first 12,000 miles of use. Often washcoated exhaust gas catalysts actually physically deteriorate. For example, when exposed to thermal shock or high thermal transients, washcoated catalysts can suffer melt-down of the support in the worst case, or sintering, spalling, or rapid deactivation due to the reaction of the catalytic metals with the support or washcoat. New government standards for catalytic converters containing exhaust gas catalysts have much stricter longevity requirements, in that such catalytic converters must perform efficiently for much longer periods of time, i.e., 50,000-100,000 miles of use.